Can multiple render systems be used?
Yes you can combine images created with two different renderer engines. These are optimistic scenarios.


Different Scenes can have different render Engines. Cycles or Blender Internal.
The [Compositor] can invoke and combine images from different Scenes.  The Render Layer Node names the scene and Render Layer.  In the nodes above the text [Blender] was rendered Blender Internal Render and the word [Cycles] was rendered with Cycles Render.  Note the transparent background for the Cycles render.
If your desired result can be composed in visual layers fine.  Only you can determine that.
Cycles has a different approach to Blender render so only you can determine what compromises you can tolerate for a coarse translation.  Just create a new material for the appropriate render engine and assign it to the faces and judge that result.
The VSE can also combine images with transparent backgrounds for the layered composition effect.